In one aspect, the invention relates to an apparatus suitable for forming a blow molded article. In another aspect, the invention relates to a method for blow molding a plastic article.
Blow molding processes are widely used to form hollow articles from plastic materials. Generally, a parison, which is a tube shaped blank of molten plastic material, is extruded from a nozzle having a generally annular opening defined between a die ring and land and is directed by gravity to between the parts of an opened mold. When a desired length of parison has been extruded, the mold is closed and compressed gas, usually air, is injected into the parison to pneumatically expand it to fit the inside contours of the mold, the article thus formed is cooled to stabilize its form, and the mold is opened to retrieve the article. When hollow articles having a cross section different substantially from the circular are to be blow molded, the sides of the circular-cross-sectioned parison are expanded a different degree so there occurs a thinning of the plastic material as the parison is stretched. Because the parison must be stretched the greatest extent in order to reach crevices, corners and edges, it is in these areas of localized concavities where weaknesses and flaws in blow molded articles are most likely to occur. The problem is especially severe where the blow molded article is to have sharp corners.
Two procedures have been used in the prior art to reduce the high article scrap rate which associates itself with blow molding articles having sharp corners. The first is to blow mold articles having rounded corners. This reduces but does not eliminate the problem since localized parison thinning occurs even in rounded corners. Also, articles having rounded corners are not necessarily as commercially desirable as articles having sharp corners since they do not pack as well. The other technique involves providing a greater amount of material to the corners and edges of the article by localized adjustment to the gap between the die ring and land. For example, providing the gap between the ring and land with a notch allows for a thickened rib of material on the parison which can be oriented with respect to the mold to provide a larger amount of material to longitudinally extending edges. By programming the distance between the die ring and land with time as the parison is dropped, thickened rings on the parison can be provided to correspond to circumferentially extending edges in the mold. While these approaches provide some benefits, processes for controlling wall thicknesses without using larger amounts of material would clearly be very desirable.